An in-core instrument (ICI) is a device for measuring the output of a nuclear reactor by measuring the density and temperature of neutron flux in a core of the nuclear reactor.
In the related art, in-core instruments were inserted into a core through the bottom of a reactor vessel, but there was a problem that the substances in the core of a reactor may leak through the hole formed through the bottom of the reactor vessel.
In order to solve this problem, all in-core instruments have been disposed close to a core through a hole at the top of a reactor vessel instead of the way of inserting them through the bottom of a reactor vessel.
The in-core instruments that are inserted through the top head of a nuclear reactor may interfere with a control rod assembly that is inserted over a fuel rod disposed at the center of the nuclear reactor.